Gas-fired fireplaces are becoming extremely popular because they are generally cleaner, safer and less expensive to build than wood-fired fireplaces. Unvented gas-fired fireplaces in particular have become increasingly popular, because they allow the consumer to fit or retrofit a fireplace into any pre-existing room having a gas line, without incurring the costs associated with chimney construction or construction of a venting system. Further, because unvented fireplaces do not require chimneys or flues, it is possible to locate the fireplace in practically any location. For example, the fireplace may be located against a wall, or in a corner, or in the middle of a room, where the flame is visible from two or more sides. The fireplace may also be of any shape and size, depending on the location in which it is to be installed.
Such fireplaces are considered to be highly efficient, drawing room air in for combustion and returning the warmed air directly to the room. However, because unvented fireplaces vent most or all of the combustion products to the directly to the room rather than outside, they must meet rigid standards, requiring that the fireplace burn extremely efficiently in order to minimize the buildup of the combustion products.
U.S. Pat. No. 2,696,205 to Ruhl describes a space heater that operates similarly to a typical unvented fireplace. The heater comprises a heater unit surrounded by a series of flues and baffles. Ambient air passes through the heater section, where it is combusted by a flame within a metal radiant element. The flow of the combustion air from the heater to the surrounding room is partially controlled by a deflecting choke surface, which accelerates expulsion of the heated air by increasing the air pressure at the choke. A second stream of air also freely flows behind the heater unit, through the flues and out the baffles. This second stream of air is warmed by thermal contact with the flues and thus also warms the room, but in addition it serves a cooling function, ensuring that the area immediately around the rear of the heater does not get too hot.
U.S. Pat. No. 5,054,468 to Moon et al. describes an unvented fireplace assembly consisting of two chambers, namely a combustion chamber surrounded by a plenum chamber. Ambient air moves through the plenum chamber, warming by contact with the outside of the combustion chamber, before returning to the room in which the fireplace is located. Air is also pulled directly into the combustion chamber by a fan; the heated combustion air is then expelled back into the room. U.S. Pat. No. 5,139,011 to Moon describes a similar unvented fireplace with a pair of airflow paths. Ambient air passes through a duct assembly around the combustion chamber, heating the air before returning it to the room. Combustion air is again pulled directly into the combustion chamber by a fan and heated before being returned to the room.
Each of these patents uses a dual intake system, which makes the system more complex, and makes it difficult to maintain a steady flow of air into both airflow pathways. If air moves relatively quickly into the combustion chamber, for example, there may not be sufficient plenum airflow to cool the area outside the combustion chamber. Further, in each of these cases, the combustion air and the plenum air are expelled into the room at different places and likely at different rates. The combustion air is hotter than the radiantly heated plenum air, which can lead to undesirable localized temperature variations.
U.S. Pat. No. 5,678,534 to Fleming and U.S. Pat. No. 5,906,197 to French each disclose a single-intake fireplace in which the incoming airflow is divided into several pathways, one flowing to the combustion chamber, and the others flowing to various convection pathways or plenum chambers surrounding the combustion chamber. The air from the plenum or convection areas is then mixed with the combustion air before being returned to the room. In order to minimize the combustion products being expelled into the room, the combustion air passes through a catalytic converter to convert any combustion products to carbon dioxide and water before the combustion air mixes with the plenum air and re-enters the room.
U.S. Pat. No. 6,425,390 to Campbell and U.S. Pat. No. 5,701,882 to Champion each disclose a gas fireplace in which combustion air is drawn directly into the combustion chamber while a second pathway for room air is provided in one or more plenum chambers surrounding the combustion chamber. Neither Campbell nor Champion disclose a combustion chamber that is essentially sealed from the surrounding room, meaning that there is little to no control over the amount of air passing into the combustion chamber. This may adversely affect the appearance of the flame and the efficiency of burning within the combustion chamber.
It is therefore an object of this invention to provide an unvented gas fireplace that overcomes one or more of the disadvantages found in the prior art fireplaces referred to above.